1. Field of the Invention
The present invention relates to an array antenna apparatus capable of sufficiently securing isolation between feeding elements and operating at a plurality of frequencies and to a wireless communication apparatus employing the same.
2. Description of the Related Art
In recent years, size reduction and thickness reduction in portable wireless communication apparatuses such as portable telephones have been rapidly promoted. Moreover, the portable wireless communication apparatuses have been not only used as conventional telephones but also achieved transfiguration as data terminal equipment for transceiving electronic mails and browsing web pages by www (World Wide Web) and so on. The information to be handled has been increased in capacity from the conventional sound and character information to photographs and motion pictures, and a further improvement in the communication quality has been demanded. Under these circumstances, an antenna apparatus using a MIMO (Multi-Input Multi-Output) technique for simultaneously transceiving wireless signals of a plurality of channels by an array antenna apparatus having a plurality of antenna elements is proposed.
As a technique for improving the coupling deterioration of an array antenna, a configuration provided with a phase shifter circuit is disclosed (See a Patent Document 1). According to the Patent Document 1, an antenna apparatus that transmits and receives radio waves of two frequencies is characterized in that the feeding points of two antenna elements having resonance frequencies different from each other are connected to a wireless circuit via respective two phase shifter circuits for changing the phase. In such an antenna apparatus, connection of an antenna element to the feeding point via the phase shifter circuit leads to that the impedance characteristic of the adjacent other antenna element at the resonance frequency can be adjusted to be high. Therefore, the influence between the antenna elements can be removed, and use at relatively adjacent frequencies different from each other is possible with a simple configuration.
As a technique for improving the coupling deterioration of the array antenna, such a configuration that the current paths of the antennas are different from each other is disclosed (See a Patent Document 2). In the Patent Document 2, an antenna apparatus having a conductive substrate of a rectangular shape and a flat plate-shaped antenna provided via a dielectric on the substrate is disclosed. The antenna apparatus is characterized in that a current flows in one diagonal direction on the substrate by excitation of the antenna in a predetermined direction, and a current flows in the other diagonal direction on the substrate by excitation of the antenna in a different direction. As described above, the antenna apparatus of the Patent Document 2 can prevent the occurrence of such a problem that the two antennas of the antenna apparatus are electromagnetically coupled with each other by changing the direction of the current flow on the substrate.
Patent and non-patent documents related to the present invention are as follows:
Patent Documents:
Patent Document 1: Japanese patent laid-open publication No. JP 2001-267841 A; and
Patent Document 2: International Publication No. WO2002/039544.
Non-Patent documents:
Non-Patent Document 1: S. Ranvier et al., “Mutual Coupling Reduction For Patch Antenna Array”, Proceedings of EuCAP 2006, Nice in France, ESA SP-626, October 2006.
However, according to the system disclosed in the Patent Document 1, the resonance frequencies of two elements are different from each other, and one antenna element becomes high impedance when used at the resonance frequency of the other antenna element. Therefore, the apparatus can not be used for the maximum ratio combining method (MRC: Maximum Ratio Combining)) for simultaneously driving two elements at an identical frequency to change the phase and the MIMO antenna apparatus. Moreover, according to the system disclosed in the Patent Document 2, it is possible to restrain such a problem that the antennas are electromagnetically coupled with each other by changing the current paths of the antennas. However, the apparatus, which is unable to perform simultaneous operation in a manner similar to that of the Patent Document 1 due to the execution of switchover, can not be used for the MRC and MIMO antenna apparatus.
Moreover, when an array antenna is provided for a compact wireless communication apparatus like a portable telephone, it is compelled to have a shortened distance between the feeding elements, and therefore, this has led to such a problem that the isolation between the feeding elements has become insufficient. Furthermore, it is desirable to provide an antenna apparatus capable of operating in a plurality of frequency bands in addition to the capability of performing the MIMO communication in order to perform, for example, communications with respect to a plurality of applications. Such an antenna apparatus has not been disclosed in the Patent Documents 1 and 2.
FIG. 29 is a plan view of a prior art array antenna apparatus disclosed in the Non-Patent Document 1. Referring to FIG. 29, patch antennas 71 and 72 are foamed on a dielectric substrate 70, and they are fed via microstrip lines 73 and 74, respectively. In this case, as indicated by arrow 76, a microstrip line 75 is connected between the microstrip lines 73 and 74 before the feeding points in order to cancel a high-frequency signal that propagates through the space from the patch antenna 71, and enters the patch antenna 72. However, there has been such a problem that the design of a spatial coupling of a reversed phase has been extremely difficult in order to cancel the high-frequency signal entering the patch antenna 72 from the patch antenna 71.